Letter of Recommendation is very short and mostly nonspecific - What should I do?

Almost all letters of recommendation for graduate program applications are short -- at my previous university, for example, there was a field that letter writers could fill in that essentially only held the equivalent of a single paragraph. Fundamentally, for current undergraduates, it is rare that a professor has spent more than a couple of hours one-on-one with you, and so it's hard to really say much of any specificity -- as would be the case if one were writing for one's own graduate student or postdocs, for example.

In other words, since almost all letters are that short, you're not suffering any harm. Go on doing what you're doing well, and don't worry about the letter too much.

Succinct isn't necessarily bad.

Rather than judging the letter by the number of paragraphs, analyze the content. Actually, before you do that, sit down and outline what you think a strong letter about you should touch on. Compare your outline and the professor's draft. If there are important aspects of you that didn't make it into the draft, let your professor know.

If the admission committee of the program you are applying to has heard of this guy, and respect his opinion, then even a letter with one line would be enough.